A conventional turbine housing 001 includes, as shown in FIG. 9, a scroll part 002, a core part 003 surrounding the outer circumference of a turbine wheel 004, and an outlet part 005 leading to an exhaust gas flow passage, these being formed integrally by casting. As it is made by casting, the turbine housing is thick in various parts and has high heat capacity, because of which, when disposed upstream of an exhaust gas purification catalyst, the housing impedes the warming of the catalyst. In view of more stringent exhaust gas regulations in recent years, it is important to reduce the heat capacity of the housing by making it from sheet metal so that it is thinner and lighter, so as to improve the purification performance of the catalyst by speeding up the warming and activation of the catalyst.
Since sheet metal parts are thin and less strong, a turbine housing made of sheet metal requires a coupling portion for coupling a part on the side of a bearing housing in which a bearing that supports a rotating shaft of the turbine blade is fitted and a part on the side of the outlet leading to the exhaust gas flow passage.
FIG. 10 shows a conventional technique regarding this coupling portion described in Patent Document 1 (Japanese Patent Application Laid-open No. 2008-106667).
A coupling ring 08 having a plurality of circumferentially arranged columnar coupling portions 081 is disclosed, as shown in FIG. 10. A flange 03 on the bearing housing side, a flange 05 on the nozzle wall surface, and the coupling ring 08 are fabricated as separate components. The coupling ring 08 and the flange 05 on the nozzle wall surface are welded together all around (W6), and the coupling ring 08 and the flange 03 on the bearing housing side are welded together all around (W7).
As the flange 03 on the bearing housing side, the flange 05 on the nozzle wall surface, and the coupling ring 08 are fabricated as separate components and welded together all around, the cost is increased because of the increased number of components. Moreover, an ample clearance needs to be formed between the housing portion and the turbine, in consideration of the amount of thermal strain that accompanies the welding of the entire circumference. A large clearance is problematic because it lowers the driving performance of the turbine, and results in poorer turbocharger performance.
Another problem is that, while the turbine housing can be made more lightweight by making it from sheet metal, a sheet metal housing will likely have lower strength. Therefore, the turbine housing needs a structure that provides impact resistance so that, in the event of a burst following an excessively high-speed rotation of the turbine rotor under some circumstances, fractured pieces will not fly off.
Patent Document 1: Japanese Patent Application Laid-open No. 2008-106667